Many Japanese corporations have invested heavily in the U.K., including global banking giant Nomura and top automakers Toyota, Nissan and Honda.

Japan published a detailed list of its concerns about Brexit shortly after Brits voted in the EU referendum in 2016.

Japanese firms wanted reassurances that the U.K. would seek to limit any "harmful effects" on their businesses, according to the government document.

It warned that those with European headquarters in the U.K. may move those functions elsewhere in Europe "if EU laws cease to be applicable in the U.K. after its withdrawal."

Japan's foreign ministry confirmed Thursday that it stands by the memo.

The top European executive from Nissan, which employs nearly 7,000 people at the U.K.'s largest car factory, attended the meeting with May, along with representatives from Hitachi, Toyota, Honda, Mitsubishi, SoftBank and Mizuho.

Urgent requests for clarity are also being made on behalf of banks and financial services companies.

Nicky Morgan, a lawmaker who chairs the influential Treasury committee in the House of Commons, accused the government of throwing the sector into a "chronic state of uncertainty."

Many companies will begin to activate contingency plans for Brexit next month, but still don't know what the government hopes to achieve in terms of access to European markets, she said in a statement.

"The clock is ticking for the financial services industry," Morgan added.

Consulting firm Oliver Wyman estimates that 75,000 finance jobs could be lost in the long term if Brexit goes badly.

The U.K. government says it believes it can achieve "as frictionless as possible" trade with the EU because doing so is in the interest of both parties.

Even if top British politicians can settle on a preferred option, there's no guarantee it will be accepted by the EU.

"This is a crucial year for the Brexit negotiations and of course we will all be better informed, we hope, by certainly this time next year about the future trading relationship with the EU," Bank of England Governor Mark Carney said Thursday.